hello!!
CNG is alternatice fuel only....it has no match with petrol....CNG just runs fine but it needs lot of cares....it does'nt burn easily bcz of its nature...spark did'nt pass out easily from methane so u need strong ignition system....+ its octaing rating is high so its benifit are no engine knocking bcz it is self ignited on 600-650 degress (celcius) where are petrol self ignites on 350 degress celcius...so CNg is a good alternative of petrol but some bigger engine gives u better result...
Thanxxx